🔧 fix: repair test, build and CI infrastructure

The vitest suite silently ran zero tests: vitest 4 transpiles with oxc, which
does not pick up `experimentalDecorators` from a tsconfig that excludes the
files being transformed, so every decorator-using suite failed to parse and was
reported as "0 test". A vitest.config.ts enabling legacy decorators brings all
40 existing tests back to life.

- Add vitest.config.ts (oxc legacy decorators + v8 coverage config)
- Type-check test files: move the test/demo exclusions from the base tsconfig
  onto the two build configs, and fix the strict-mode errors this surfaced
- Stop shipping src/example.ts in dist (it invokes runExample() at import time,
  a side effect in a package declaring "sideEffects": false)
- CI: run lint, tests with coverage, build and entry-point smoke checks; drop
  EOL Node 18, add Node 24; commit package-lock.json so `npm ci` works
- Add `verify`, `test:watch` and `build:docs` scripts, and an engines field;
  `build:docs` regenerates the previously hand-maintained docs/cereale.js

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SAcqrz3FcadkYr3xG32CjK

.github/workflows/ci.yml

@@ -7,12 +7,14 @@ on:
     branches: [ main ]
 
 jobs:
-  build:
+  verify:
+    name: Node ${{ matrix.node-version }}
     runs-on: ubuntu-latest
 
     strategy:
+      fail-fast: false
       matrix:
-        node-version: [18.x, 20.x, 22.x]
+        node-version: [20.x, 22.x, 24.x]
 
     steps:
     - uses: actions/checkout@v4
@@ -25,5 +27,15 @@ jobs:
       run: npm ci
     - name: Type Check
       run: npm run type-check
+    - name: Lint
+      run: npm run lint
+    - name: Test
+      run: npm run test:coverage
+    - name: Build
+      run: npm run build
+    - name: Verify published entry points load
+      run: |
+        node --input-type=module -e "import * as m from './dist/esm/index.js'; if (typeof m.toInstance !== 'function') throw new Error('ESM entry point broken');"
+        node --input-type=commonjs -e "const m = require('./dist/cjs/index.js'); if (typeof m.toInstance !== 'function') throw new Error('CJS entry point broken');"
     - name: Run Demo
       run: npm run demo